Flexography is a method of printing whereby a flexible plate with a relief image is wrapped around a cylinder and its relief image is inked up and the ink is then transferred to a suitable printable medium. The process has mainly been used in the packaging industry where the plates should be sufficiently flexible and the contact sufficiently gentle to print on uneven substrates such as corrugated cardboard as well as flexible materials such as polypropylene film. The quality of the printing in this manner is inferior to processes such as lithography and gravure, but nevertheless it is useful in certain markets. In order to accommodate the various types of printing media, the flexographic plates should have a rubbery or elastomeric nature whose precise properties can be adjusted for each particular printable medium.
In addition, where the flexographic printing plates are formed and/or imaged in a flat form, they should be flexible for bending around a cylinder for rotary printing. This can present more of a problem than with offset lithographic plates because the thickness of flexographic printing plates is generally several millimeters instead of fractions of a millimeter. Materials that are flexible as one or two μm films can be rigid and inflexible at one or more mm.
U.S. Pat. No. 4,323,636 (Chen) describes elements having thermoplastic elastomeric block copolymers (for example, those sold by Kraton Polymers under the trademark of KRATON) used in conjunction with an acrylate or methacrylate monomer and a photoinitiator. The upper surface may have on it a thin hard flexible solvent insoluble coating and on top of this a strippable thin film of e.g. polyethylene to protect the plate during storage. This would constitute a flexographic printing plate precursor that can be imaged by ultra-violet exposure through a negative mask, and the un-polymerized material washed away with solvent. Such elements usually have a thickness of one or more millimeters. Exposure from the front through an image bearing transparency is sufficient to polymerize both the image areas and the underlying layer. The block co-polymer materials formulated as printing blanks, whilst being able to be formed into solid blanks, retain a stickiness on their upper and lower surfaces that requires the use of protective films to prevent unused plates sticking together during storage.
U.S. Pat. No. 4,994,344 (Kurtz et al.) describes flexographic printing blanks prepared from ethylene-propylene-alkadiene terpolymers. It describes the process of initial back exposure to establish the “floor” of the plate before image exposure from the front of the plate through a negative mask. The image floor may be uneven due to differences in evenness of UV exposure and subsequent wash-out, but this would be of little consequence where image thickness (relief) is measured in millimeters. Because the plate requires back exposure, a polyester substrate is often used as the dimensionally stable backing material and must be transparent to UV light. In addition, the floor material of the plate is of the same material and formulation as the image areas. Thus, the finished plate generally has little or no visual contrast between the image areas and the floor and it is difficult for the user to make any visual assessment of the image because of this lack of contrast.
U.S. Pat. No. 5,719,009 (Fan) describes the use of a negative mask that is integral in the flexographic printing plate itself. The flexographic printing plate comprises photosensitive layers and an overcoat containing carbon black with a binder resin. The overcoat is ablated with an infrared laser in response to a digital signal generated by a computer. Digital imaging using a modulated laser source is an important part of the general technology that has become known as computer-to-plate (CTP) and is used for instance in the production of offset lithographic printing plates. The ablated areas in the overcoat permit a subsequent irradiation by UV light to expose the sensitive elastomeric layer and to harden it. The other unexposed areas situated under the non-ablated areas of the overcoat are washed away together with the remains of the carbon layer, leaving a relief image. In this case, there is good visual contrast between the masked areas that remain after ablation and the image areas that are exposed by ablation. However, after UV exposure and subsequent wash-out processing, any visual contrast disappears as the overcoat is washed away with the underlying unexposed background areas, so that the imaged flexographic printing plate has no visual contrast between the image and the background. The UV exposure and washing process still results in unevenness of the image floor.
It has long been recognized that the simplest way of making a flexographic printing plate would be by direct engraving using laser beam ablation, thereby eliminating all need for washing or drying the plate or multiple types of exposure.
U.S. Pat. No. 3,549,733 (Caddell) describes the formation of a laser engraved (or imaged) relief printing plate. However, the described plates do not have the elastomeric properties needed for flexographic printing but could be used in letterpress printing. Letterpress printing differs significantly from flexographic printing in that it is more like lithography in the complexity of the printing machine and the type of ink used. Letterpress inks must have high viscosity (paste-like), similar to offset inks and do not in general contain volatile solvents. If the letterpress printing is carried out using an offset blanket, the printing process is termed dry offset. As with offset printing, dry offset and letterpress require high pressure between the plate and blanket or printable media to achieve good ink transfer, whereas flexographic printing uses the minimum pressure possible. Thus a letterpress plate would be unsuitable for flexographic printing as it would not give good ink transfer under low pressure and similarly a flexographic plate would be unsuitable for letterpress as the high pressure would distort the softer plate and give very poor image quality with huge dot gain.
U.S. Pat. Nos. 5,798,202 and 5,804,353 (both Cushner et al.) describe the use of single or multiple layers of elastomers in flexographic printing plate precursors for direct laser engraving. The upper layer of the precursors is comprised of a thermoplastic elastomeric material.
Imaging sensitivity is limited by the use of large quantities of block polymers, such as those sold under the trade name of KRATON by Kraton Polymers. Poor melt edges are reported for flexographic engraving of layers containing such polymers in U.S. Pat. No. 6,627,385 (Hiller, in the Comparative Examples). The patent also describes the problem of using carbon black or opaque fillers in that the flexographic printing plate loses its transparency, which complicates mounting it with accurate register, since register crosses or similar marks, are no longer visible through the plate. Hiller suggests avoiding such layers.
U.S. Pat. No. 6,159,659 (Gelbart) describes a flexographic printing plate precursor having two ablatable layers, the upper layer comprising an elastomeric foam mounted on a thin non-ablatable backing layer where preferably ablation removes material right down to the backing layer. The method is intended to solve the problem in the prior art of small holes and nicks in the backing caused by exposure by the laser that reduce the life of the printing plate. However, no attempt is made to provide very high adhesion between the two layers that may be needed especially for small isolated image areas.
Problem to be Solved
Despite the limitations of carbon dioxide lasers, they are now being used commercially in flexographic engraving machines. They are known for slow and expensive imaging with limited resolution. However, the advantages of direct engraving are sufficient to ensure their commercial use in instances where fast imaging and high print quality are not required. It would be preferable to use infrared diodes that produce radiation in the near infrared and infrared (approximately 700 to 1200 nm) and have the advantages of high resolution and relatively low laser cost so that they can be used in large arrays. Until now, although the use of such lasers is described in many publications, they are not in industrial use because even when combined with the most sensitive imageable elements available, satisfactory engraving may not be achieved.
Infrared diode engraving (or ablative imaging) differs from that of carbon dioxide in that a compound absorbing suitable radiation (that is, IR radiation) is usually incorporated into the imaged coating. The use of an organic infrared radiation-sensitive dye (IR dye) may be prohibitive because it is costly and a large quantity of IR dye is needed throughout the imaging layer (which may be millimeters thick). The use of an opaque pigment such as carbon black reduces the possibility of visual contrast even further. Another problem experienced with high carbon and other fillers is the loss in layer resilience. Good resilience ensures the rapid elimination of any distortion of the plate during a printing cycle by permitting the plate to recover its original shape in time for the next cycle. Distortion may also occur from dirt entering the printing system and causing temporary indentations in the printing plate surface. Thus, good resilience is needed to provide fast recovery from distortion of any type.
The recent availability of high power (for example, 8 watts) IR-laser diodes opens up opportunity for the use of relatively low cost laser diode arrays capable of engraving flexographic blanks as described in WO 2005/84959 (Figov). Relief depth in the resulting image is an issue with laser engraving because the deeper the required relief, either more power is required or it takes longer to engrave or image the plate. Besides the segment of the flexographic market that involves deep relief, laser engraving is most suitable and competitive when applied to the high quality market segment using even substrates where low relief is a distinct advantage. In this market segment, it is of no advantage to work with high relief images as the image areas would have the possibility of more movement during printing and more dot gain and inaccuracy of printing. However, in order to achieve minimum image relief, the floor of the image should be extremely even, otherwise there is the danger that any slightly elevated floor area would give unwanted background printing.